1. Field of the Invention
The present invention relates to dense wavelength division multiplexing (DWDM) thin film filters, and particularly to the composition of layers of high refractive index thin films of such thin film filters.
2. Description of Related Art
A DWDM thin film filter comprises a multi-cavity film stack which is deposited on a glass substrate.
U.S. Pat. No. 6,215,592 discloses an optical thin film filter having broad resonant frequency passbands for filtering an optical input, including a plurality of multiplexed optical wavelengths in a first set of transmitted wavelengths and a second set of reflected wavelengths. The filter has first and second inner mirrors separated substantially by an inner spacer, a first outer mirror separated from the first inner mirror substantially by a first outer spacer, and a second outer mirror separated from the second inner mirror substantially by a second outer spacer. The inner mirrors have a reflectivity which is greater than the reflectivity of the outer mirrors. Each inner mirror comprises dielectric layers of high refractive index material and dielectric layers of low refractive index material, alternately deposited one on another to form a stack. In the optical thin film filter, each of the layers in the first and second inner mirrors and in the first and second outer mirrors is generally comprised of one of the following materials: silicon dioxide (SiO2), tantalum pentoxide (Ta2O5), titanium dioxide (TiO2), aluminum oxide (Al2O3), hafnium dioxide (HfO2), and zirconium dioxide (ZrO2).
The odd and even numbered standard ITU channels of the above-mentioned optical thin film filter are separated by a frequency spacing of 200 GHz. Modem optical thin film filters are increasingly being required to have channel spacings of 100 GHz, 50 GHz or even less. Accordingly, larger numbers of cavities in optical filters are required to meet the increasingly demanding requirements for pass bandwidth and isolation bandwidth. Internal stress is intrinsic to multilayer optical thin film filters and to the film deposition process involving large numbers of cavities in a film stack. Ever larger numbers of cavities increases internal stress of an optical thin film filter. This results in higher rates of failure during manufacture and in use, and unacceptably high insertion loss of the manufactured optical thin film filters.